1. Field of the Invention
The invention relates to a hydraulic control system of a 4-speed automatic transmission for vehicles which can perform a skip shift from fourth speed stage to second speed stage and enhance a power performance and a shift-responsiveness by means of a variable control of the line pressure.
2. Description of a Prior Art
The automatic transmission for vehicles comprises a torque converter, a shifting gear mechanism for multiple speed stages connected with the torque converter, and hydraulic-operated friction elements for selecting a speed stage according to the vehicle's travelling condition.
The torque converter includes an impeller connected to the engine crank shaft and rotating therewith, a turbine splined to the input shaft of the shifting gear mechanism and opposite to the impeller and a stator for changing the flow direction of the oil, located between the impeller and turbine.
An oil pump is provided in the input shaft of the transmission and hydraulic pressure is produced when the engine operates. The pressure is provided to each shift valve by the working of a manual valve connected with the select lever and selectively operates the friction elements according to the vehicle speed and the opening of the throttle valve.
The known hydraulic control system for the 4-speed automatic transmission provides the hydraulic lines only with the two mode pressures, i.e., the "D" range pressure and "R" range pressure, and the oil pump experiences the drive loss and the fuel consumption ratio is deteriorated.
Further, the hydraulic control system cannot perform a skip shift and the shift-responsiveness is sluggish.
A korean patent application No. 94-7336 suggests a hydraulic control system for solving the above problems.
The hydraulic control system performs the skip shift from fourth speed stage to second speed stage, however, it goes via temporarily third speed stage so that it takes long time for skip shift and the shift-responsiveness is slow.